The PB&J with banana is defined by the one ingredient that breaks the original sandwich's whole engineering. A plain peanut butter and jelly is built to survive: peanut butter waterproofs the bread, jelly is the only wet element, and the thing can sit in a bag for hours without failing. Add banana and that ceasefire ends. Banana is soft, wet, and actively oxidizing, a fresh-fruit element that the peanut butter cannot fully seal against, so the sandwich stops being the indestructible lunchbox staple and becomes something that wants to be eaten soon. That trade is the entire point. It gives up shelf life to gain a third texture and a mellow, custardy sweetness that jelly's bright acid never delivers on its own.
The craft is in defending the bread against the new arrival. Peanut butter still goes edge to edge on both slices, but now it is doing double duty: it has to hold back the jelly from one side and the banana's moisture from the other, so a thicker, firmer layer earns its keep. The banana is sliced into coins thin enough to mash slightly under pressure and lock into the peanut butter rather than roll out the side, and it goes in just before eating because every minute it spends cut is a minute it darkens and weeps. The jelly is often pulled back or dropped entirely, since the banana already supplies sweetness and the two together can tip the sandwich into cloying. The bread stays soft and faintly sweet, the same yielding carrier the original demands, because a fruit-loaded filling with no chew of its own needs a crumb that does not fight it.
This is the fruit branch of the peanut butter family, and it is one stop short of a different sandwich entirely. Griddled in butter it turns hot and molten, the banana caramelizing into the peanut butter. Add bacon and it becomes the fried, savory build associated with Elvis, which is its own thing and not this one. The honey-and-banana version and the marshmallow-creme Fluffernutter run parallel sweet logic. Those deserve their own articles rather than being crowded in here.